Switching devices arc well known in the field of medium and high voltage switching applications. They are predominantly used for interrupting a current, when an electrical fault occurs. As an example, circuit breakers have the task of opening contacts and keeping them far apart from one another in order to avoid a current flow even in case of high electrical potential originating from the electrical fault itself. The electrical switching devices, like said circuit breaker, may need to carry high nominal currents of 3 kA to 6.3 kA. They may switch very high short circuit currents of 31.5 kA to 80 kA at very high voltages of 72 kV to 1200 kV (high current duty, such as SLF90 duty). Circuit breakers may also need to perform low short-circuit current duty, such as T10, T30 and out-of-phase duty, up to about three times the nominal currents, e.g. from 9 kA to 15 kA. The operation principle of circuit breakers is known and will not be described here in detail.
Such electrical switching devices like circuit breakers comprise an arcing contact arrangement used for taking over the current from the nominal contact(s) during the opening and closing operation of the device. Amongst others, one type of circuit breakers uses a tulip-shaped arcing contact, comprising contact fingers arranged concentrically around a longitudinal axis of the circuit breaker. This arcing configuration is called a contact tulip. The mating arcing contact is a pin or rod or a tube, which is inserted into the contact tulip during a closing operation of the switching device. An auxiliary nozzle encloses at least partially the contact tulip. A main nozzle at least partially encloses the auxiliary nozzle.
Circuit breakers are designed in a way that the insulating and cooling gas is accelerated effectively in the nozzle system. In high current duty, the flow between the main nozzle and auxiliary nozzle should reach sonic conditions on either side of the stagnation point at a comparatively short distance from it and then accelerate to supersonic speed. This flow pattern corresponds to an effective convective cooling of an arc and favors the interruption of the conductive path.
However, in low short-circuit current duties, such as T10, T30 or out-of-phase duty, the circuit breaker may exhibit a low dielectric withstand, as such flow conditions may not be reached. Therefore, there is a need to improve the dielectric withstand of gas-insulated high-voltage switching devices, such as gas-insulated high-voltage current breakers, particularly with respect to low short-circuit current duties.
DE 10 2011 007 103 A1 discloses a circuit breaker having a heating channel for guiding arc-heated gases to a heating volume and back again to the arcing zone. The heating channel smoothly merges into the nozzle throat. The nozzle opens divergently, when starting from the heating channel entrance and moving axially away from the arcing zone. Moreover, In this design the main nozzle throat has zero length in axial direction.